1. Field of the Invention
The present invention relates generally to a channel communication method in a mobile communication system, and in particular, to a method for performing a handover and mode switching using an uplink synchronous transmission scheme (USTS).
2. Description of the Related Art
A CDMA (Code Division Multiple Access) mobile communication system is divided into a synchronous system and an asynchronous system. Such a CDMA communication system uses orthogonal codes to separate channels. Herein, a description of the invention will be made with reference to an asynchronous W-CDMA (Wideband-CDMA) communication system, also known as a UMTS (Universal Mobile Telecommunications System) communication system. However, the invention can also be applied to different CDMA systems such as the CDMA-2000 system, as well as the W-CDMA system.
The W-CDMA communication system employs an uplink synchronous transmission scheme (USTS) in which a first Node B communicates with a plurality of UEs (User Equipments) through radio links formed between them while maintaining an orthogonal property among the signals received from the respective UEs. For the USTS, the Node B transmits a control signal to the UEs so that the respective UEs can transmit their signals at a time proper to maintain the orthogonal property among the UEs. Upon receipt of the control signal, the UEs align a transmission time of the uplink signals.
FIG. 1 illustrates architecture of a conventional W-CDMA communication system. As illustrated, a radio network controller (RNC) controls a process for connecting the UE. Further, the RNC manages assignment of channel resources to the UEs connected to one or more Node Bs. The Node Bs and the RNC constitute a UTRAN (UMTS Terrestrial Radio Access Network).
When successfully connected to the Node B through the channel assigned by the RNC, the UE maintains communication using the assigned downlink or uplink dedicated physical channel (DPCH). The RNC can communication with a plurality of UEs through the Node B. In this case, the UE scrambles its transmission data using its unique scrambling code and transmits the scrambled data as an uplink signal, so that the Node B can distinguish the uplink signals received from the respective UEs by descrambling each uplink signal with UE's unique scrambling code.
The scrambling code is classified into a long scrambling code and a short scrambling code. In the following description, the “scrambling code” will refer to the long scrambling code.
The scrambling code is created in the following process of:
(Step 1) receiving 24 initial values n0,n1, . . . ,n23,
(Step 2) creating sequences x(i) and y(i), where i=0, . . . , 225−27, x(0)=n0, x(1)=n1, x(2)=n2, . . . , x(23)=n23, x(24)=1 x(i+25)=x(i+3)+x(i) modulo 2, i=0, . . . , 225−27 y(0)=y(1)=y(2)= . . . =y(23)=y(24)=1 y(i+25)=y(i+3)+y(i+2)+y(i+2)+y(i) modulo 2, i=0, . . . , 225−27.
(Step 3) creating a sequence z(i), where i=0, . . . , 225−2, z(i)=x(i)+y(i) modulo 2, i=0, . . . , 225−2,
(Step 4) creating a Gold sequence Z(i), where i=0, . . . , 225−2, Z(i)=1−2*z(i)
(Step 5) creating two real scrambling codes c1(i) and c2(i), where i=0, . . . , 225−2, c1(i)−Z(i) c2(i)=Z((i+16777232) modulo (2225−1)),
(Step 6) creating a scrambling code C(i), where i=0, . . . , 225−2, C(i)=c1(i)*(1+j(−1)i*c2(2*└i/2┘)).
In the above formula, └i/2┘ indicates the largest integer less than or equal to i/2.
In the W-CDMA communication system, one frame is comprised of 38400 chips. Therefore, the scrambling code is used in a unit of 38400 chips. That is, a scrambling code for one DPCH is C(i), where i=0,1, . . . ,38399.
A single DPCH frame signal is scrambled using the scrambling code C(0) to C(38399). The respective UEs create its unique scrambling code using different initial values n0,n1, . . . ,n23, and then, scramble the DPCH signal with the created unique scrambling code before transmission. The Node B then descrambles the signals received from the UEs using each unique scrambling codes uniquely assigned to the respective UEs, thereby distinguishing the signals from the respective UEs.
The latest W-CDMA communication system uses OVSF (Orthogonal Variable Spreading Factor) codes for down link channel separation. In the case of the downlink, the Node B can distinguish the downlink DPCH (DL DPCH) signals transmitted to the different UEs using different OVSF codes. The Node B spreads the DL DPCH signals using the OVSF codes uniquely assigned to the respective UEs, sums up the spread DL DPCH signals, scrambles the summed DL DPCH signal with its unique scrambling code, and then transmits the scrambled DL DPCH signal. The respective DPCHs may have different data rates. In the case of the uplink, the UE spreads a uplink DPDCH (Dedicated Physical Data CHannel) signal and a DPCCH (Dedicated Physical Control CHannel) signal constituting a DPCH signal, using different OVSF codes, and scrambles the spread DPDCH and DPCCH signals with its unique scrambling code before transmission. The OVSF codes used by the UE to spread the DPDCH and DPCCH signals may also be its identical to each other. Since the UEs transmit the signals using the different scrambling codes, the Node B can distinguish the signals received from the respective UEs.
The UE employing the USTS scheme spreads the DPDCH signal and the DPCCH signal with its unique OVSF codes and scrambles the spread DPDCH and the spread DPCCH signal with an uplink scrambling code commonly-used by the UEs in a cell where it is located, instead of using its unique scrambling code, and transmits the scrambled signals. That is, the UE employing the USTS scheme spreads DPDCH signal and the DPCCH signal with its unique OVSF code, i.e., a channelization code, assigned from the Node B. The Node B then despreads the signals received from the respective UEs using the OVSF codes uniquely assigned to the Ues and the common scrambling code, thereby distinguishing the received signals.
In addition, the W-CDMA communication system transmits the respective DL DPCH signals with different time offsets, in order to prevent the transmission power from increasing instantaneously when the Node B simultaneously transmits a plurality of downlink DPCH (DL DPCH) signals. By doing so, the uplink DPCH (UL DPCH) signals also arrive at the Node B at different points in time, preventing the Node B from simultaneously processing the signals received from a plurality of UEs, thereby distributing a load of the Node B.
FIG. 2 illustrates the timing relationship between the DL DPCH signal and the UL DPCH signal in the W-CDMA communication system, wherein it is assumed that there is no propagation delay between the Node B and the UEs, i.e., that the UE receives the DL DPCH transmitted by the Node B with no propagation delay and the Node B also receives the UP DPCH transmitted by the UE with no propagation delay. When there exists a propagation delay between the Node B and the UEs, a round trip time (RTT) must be considered. However, since the system will operate in the same manner even though there exists the propagation delay, the round trip time can be assumed to be ‘0’ for simple explanation.
Referring to FIG. 2, one 10-ms frame is comprised of 15 slots, and each slot is comprised of 2560 chips. A common pilot channel (CPICH) signal and a primary common control physical channel (P-CCPCH) signal are frame-synchronized with each other, and used as a reference system time for other channels.
As illustrated in FIG. 2, the respective DL DPCH signals are transmitted with a time offset τDPCH,n from the P-CCPCH signal(reference system time). The respective DPCH signals are given the different time offsets τDPCH. For example, each DPCH signal is given one of 0, 256, 2*256, . . . , 148*256 and 149*256-chip offsets. That is, the DPCH signal is given a time offset of a multiple of 256 chips from the reference system time.
The UE transmits the UL DPCH signal after a lapse of a To time after receiving the DL DPCH signal with a time offset τDPCH,n from the P-CCPCH signal(reference system time). Therefore, the UL DPCH signals also have different transmission time points, so that the UL DPCH signals arrive at the Node B at the different time points. Due to a distance difference between the Node B and the respective UEs, the Node B may not receive the UL DPCH signal exactly after a lapse of the To time after transmitting the DL DPCH signal. Therefore, the Node B measures a propagation delay time to the UE in the process of a random access channel (RACH) signal in order to measure a distance from the UE, and uses this value(propagation delay time) in predicting an expected UL DPCH signal arrival time after transmission of the DL DPCH signal.
In the USTS mode, a plurality of UEs can communicate with a Node B using the same scrambling code. The USTS scheme is designed to synchronize the UL DPCH signals received at the Node B from a plurality of UEs. In the USTS mode, the Node B assign the same scrambling code to the USTS mode UEs. Therefore, the W-CDMA communication system employing the USTS scheme can reduce the number of scrambling codes used in the cell, contributing to a reduction in interference between UE signals. When the UEs employing the USTS use the same scrambling code, the Node B can identify the UE signals using channelization codes, i.e., the OVSF codes provided from the RNC(Radio Network Controller). In the USTS mode, the RNC assigns one scrambling code for USTS UEs and assigns different OVSF codes(channelization codes) to each UEs the Node B synchronizes the UL DPCH signals from at least 2 UEs with each other, and then assigns the same scrambling code to the synchronized UEs. Further, the Node B assigns the different channelization codes (or OVSF codes) to the UL DPCHs of the UEs assigned the same scrambling code, to distinguish the received synchronized UL DPCHs.
The USTS controls a sync time of the signal through the following two processes.
(1) Initial Synchronization Process
Upon receipt of a signal from a UE over the RACH, a Node B measures the difference between a predetermined reference time and an arrival time of the signal received over the RACH. The Node B transmits the measured time difference to the UE over a forward access channel (FACH). Upon receipt of the time difference over the FACH, the UE aligns its transmission time using the received time difference.
(2) Tracking Process
The Node B transmits a time alignment bit (TAB) to the UE by periodically comparing the arrival time of the UE signal and the reference time. If the TAB is ‘1’, the UE shifts the transmission time ahead by ⅛ chip. However, if the TAB is ‘0’, the UE shifts the transmission time behind by ⅛ chip. The TAB is transmitted once every two frames using a transmit power control (TPC) bit in the control channel.
In the USTS mode where several UEs use the same scrambling code, the uplink frame signals transmitted by the UEs using the same scrambling code must be synchronized with one another at the Node B. That is, when the Node B receives the DPCHs transmitted from several UEs, the received DPCHs must be subjected to both slot synchronization and frame synchronization. The frame synchronization is to minimize interference among the UEs using the same scrambling code, while the slot synchronization is to distinguish the UE signals from each other. The UEs perform spreading using the different OVSF codes and perform scrambling using the same scrambling code, depending on the orthogonal property of the OVSF codes. The Initial Synchronization Process is a process for acquiring the frame synchronization and the slot synchronization.
As described above, the respective DL DPCHs have different time offsets τDPCH,n. Therefore, the UL DPCHs received at the Node B are not synchronized (or misaligned) with one another. During the Initial Synchronization Process, the misalignment among the UL DPCHs is removed to synchronize the UL DPCHs. Accordingly, there is a demand for a method for resolving the channel misalignment problem in the Initial Synchronization Process.
As stated above, since the USTS synchronizes the uplink within one cell and uses the channelization codes and a specific scrambling code different from a normal DPCH not supporting the USTS service, a special handover method is required. That is, in the case of the normal DPCH, each UE uses a unique uplink scrambling code. However, in the case of the USTS, a plurality of the UEs share the same scrambling code. Further, in the case of the normal DPCH, a node position of an OVSF code for spreading the DPCCH signal is SF256 which is the highest position in the OVSF code tree. However, in the case of the USTS, the node position may be different. In addition, a node position of the OVSF code for spreading the DPDCH may also be different from the node position of the OVSF code in the OVSF code tree, used by the normal DPCH. However, in the case of the USTS, the UE performs special synchronization, so that when the handover is performed in the same method as done by the existing UMTS system, two or more connections operate differently. Therefore, it is not possible to perform a USTS handover using the existing handover method. Thus, there is a demand for a separate handover method.
In addition, there is a demand for a mode switching method for switching an operation mode of the UE to the USTS mode, when the UE, which supports the USTS service but operates in the normal mode or non-USTS mode, enters into coverage of the Node B supporting the USTS service.